The distribution of braking force in a conventional vehicle relates to the ratio between the front and rear axle braking force.
This ratio cannot be affected by the driver, since he brakes both axles with a single brake pedal, so that, in the design of vehicle brakes, the braking force distribution has to be very deliberately established.
The purpose of an ideal braking force distribution consists on the one hand in braking the front and rear axles with equal force (with respect to the dynamic axle loads), and on the other hand in selecting the distribution when braking on curves such that a neutral performance is achieved. In this case the aim of a neutral performance is secondary to the equal braking force.
The equal braking force (with respect to the dynamic axle loading) front and rear can be achieved in straight-line driving. On curves (particularly at the edge of the curve), the braking of the rear axle must be made lower than at the front axle, to provide good handling.
The reason is that, in spite of the axle load shift, the centrifugal force acts at the center of gravity of the vehicle, so that the rear axle has to transfer a higher lateral guidance force with respect to the dynamic axle load than the front axle. This higher lateral guidance force is achieved by reduced braking force.
The ideal braking force division between front and rear depends on the following factors:
(1) static weight distribution and location of center of gravity (at the current load); PA0 (2) longitudinal slowing of the vehicle; PA0 (3) transverse acceleration of the vehicle; PA0 (4) engine reverse torque; PA0 (5) upgrade/downgrade. PA0 (1) poor braking distance, as long as the front wheels do not lock up, or in the case of ABS control (especially with the vehicle loaded); PA0 (2) heavy stress on the front wheel brakes (lining wear), large brakes need more space; PA0 (3) limited steerability due to heavy braking of front wheels; PA0 (4) unstable braking on turns is possible if the following factors are involved:
In conventional brakes the braking force distribution is tuned independently of these factors; it consists of a fixed ratio between front and rear and is brought about by the fact that the same hydraulic pressure acts both front and rear on different size brakes.
The braking force distribution is selected in accordance with the principle that the rear axle must not lock before the front axle. The reason for this requirement is that, if the rear axle of the vehicle is too heavily braked the vehicle can become unstable in braking on turns, i.e., it can lead to lateral skidding.
Disadvantages of the conventional braking force distributions are:
(a) reverse engine torque; PA1 (b) downhill driving; PA1 (c) great deceleration (above the intersection of the ideal distribution with the conventional); PA1 (d) brake force distribution designed for high rear axle braking (the distribution required by the regulator for straight travel can nevertheless be satisfied); PA1 (e) variations in the brake pad friction on the brake disk; PA1 (f) variations in the brake system (hot front wheel brake, fading).
If in a vehicle the ideal braking force distribution depends greatly on the longitudinal deceleration of the vehicle, a rear axle pressure limiter or reducer is normally used.
If in a vehicle the ideal braking force distribution depends also to a great extent on the loading, a load-related rear axle pressure reducer or limiter is commonly used.
These limiters or reducers are certainly necessary for many vehicles, but they basically impair stability when braking on curves (in comparison to conventional fixed tuning, i.e., to unbroken straight lines).
Another problem is that the pressure reducers and limiters can lose their function due to a defect, without the defect being noticed by the driver (their operation isn't even tested in routine servicing).